It has been confirmed by high-resolution x-ray photoelectron spectroscopy that the Fermi level of K-contacted sulfur-passivated GaAs(100) is not located at the Fermi level of the ideal Schottky contact and is pinned near the midgap, even though the initial S passivation removes the high density gap states. With K deposition the Fermi level moves to 0.5 eV above the valence band maximum for both n- and p-type surfaces. The interfacial chemical reaction shows the top As element preferentially reacts with deposited K, and the interface still has dominant Ga-S bondings. These results imply that although the reactive alkali metal such as K does not break Ga-S bondings, it induces the metallic state which pins the Fermi level at 0.5 eV above the: valence band maximum.